How Aleo Uses Leo to Simplify Zero-Knowledge Programming

Prateek Tripathi
5 min readNov 3, 2023

If you are a developer interested in building private applications on the blockchain, you may have heard of Aleo, a platform that enables fully private and verifiable computation on the web. Aleo leverages zero-knowledge proofs, a cryptographic technique that allows one party to prove to another that a statement is true, without revealing any additional information.

Zero-knowledge proofs are powerful tools for enhancing privacy, security, scalability, and interoperability of blockchain applications. However, they also pose significant challenges for developers, who have to deal with complex and low-level cryptographic concepts and protocols. This can make zero-knowledge programming difficult, error-prone, and time-consuming.

That’s why Aleo introduces Leo, a new programming language designed specifically for writing private applications with zero-knowledge proofs. Leo is a functional, statically typed language that abstracts low-level cryptography and makes it easy to express logic in zero-knowledge. Leo offers developers a high-level, blockchain-native syntax, robust error handling, and a “private by default” safety model, combining performance, reliability, and security in blockchain development.

In this article, we will explain what Leo is, how it works, and what benefits it brings to developers and users.

What is Leo?

Leo is a programming language for private applications on Aleo. It is inspired by traditional languages like JavaScript, Scala, and Rust, with a strong emphasis on readability and ease-of-use. Leo allows developers to write concise and expressive code that can be compiled to zero-knowledge circuits, which are the building blocks of zero-knowledge proofs.

Leo has several features that make it suitable for zero-knowledge programming, such as:

  • Functional paradigm: Leo is a functional language, which means that it treats computation as the evaluation of mathematical functions and avoids changing state and mutable data. This makes Leo code easier to reason about, test, and debug, as well as more compatible with zero-knowledge circuits, which are also functional in nature.
  • Static typing: Leo is a statically typed language, which means that it checks the types of variables and expressions at compile time, rather than at run time. This helps Leo catch errors early, enforce type safety, and optimize performance. It also helps Leo generate efficient and secure zero-knowledge circuits, which require precise and consistent types.
  • Private types: Leo supports private types, which are special types that can be used to declare private variables and functions. Private types are encrypted and hidden from the public, and can only be accessed by authorized parties. Private types enable Leo to implement a “private by default” safety model, which ensures that sensitive data and logic are always protected by zero-knowledge proofs.
  • Circuit generation: Leo can automatically generate zero-knowledge circuits from Leo code, without requiring developers to write any low-level cryptography or deal with any complex protocols. Leo can also optimize and verify the generated circuits, ensuring that they are correct, efficient, and secure.

How does Leo work?

Leo works by compiling Leo code to zero-knowledge circuits, which can then be used to generate and verify zero-knowledge proofs. A zero-knowledge circuit is a mathematical representation of a computation that can be executed in zero-knowledge, meaning that it can prove that the computation was done correctly, without revealing any inputs or outputs.

A zero-knowledge circuit consists of three main components:

  • Inputs: These are the values that are fed into the circuit, such as public or private variables, constants, or parameters. Inputs can be either witnesses or public inputs. Witnesses are private inputs that are known only to the prover, who generates the proof. Public inputs are public inputs that are known to both the prover and the verifier, who verifies the proof.
  • Outputs: These are the values that are produced by the circuit, such as public or private variables, constants, or return values. Outputs can be either public outputs or private outputs. Public outputs are public outputs that are revealed by the proof. Private outputs are private outputs that are hidden by the proof.
  • Gates: These are the basic operations that are performed by the circuit, such as arithmetic, logical, or bitwise operations. Gates can be either linear or nonlinear. Linear gates are gates that can be expressed as linear combinations of inputs and outputs, such as addition, subtraction, or multiplication by a constant. Nonlinear gates are gates that cannot be expressed as linear combinations of inputs and outputs, such as multiplication, division, or exponentiation.

Leo can generate zero-knowledge circuits from Leo code by following these steps:

  • Parsing: Leo parses the Leo code and converts it into an abstract syntax tree (AST), which is a tree-like representation of the code structure and semantics.
  • Type checking: Leo checks the types of the variables and expressions in the AST, and ensures that they are consistent and compatible. Leo also infers the types of the inputs and outputs of the circuit, and assigns them to witnesses, public inputs, public outputs, or private outputs.
  • Circuit generation: Leo traverses the AST and translates each statement and expression into a corresponding gate or a sequence of gates. Leo also optimizes the generated gates, by eliminating redundant or unnecessary gates, and simplifying complex or expensive gates.
  • Circuit verification: Leo verifies the correctness and security of the generated circuit, by checking that it satisfies certain properties, such as completeness, soundness, and zero-knowledge. Completeness means that the circuit can always produce a valid proof if the statement is true. Soundness means that the circuit can never produce a valid proof if the statement is false. Zero-knowledge means that the circuit does not reveal any information beyond the validity of the statement.

What benefits does Leo bring to developers and users?

By using Leo, developers and users can enjoy the following benefits:

  • Simplicity: Leo simplifies zero-knowledge programming by abstracting low-level cryptography and making it easy to express logic in zero-knowledge. Developers can write concise and expressive code that can be compiled to zero-knowledge circuits, without requiring any cryptographic expertise or experience. Users can interact with private applications that are powered by zero-knowledge proofs, without requiring any technical knowledge or trust.
  • Performance: Leo improves the performance of zero-knowledge applications by generating efficient and secure zero-knowledge circuits, without requiring any manual optimization or verification. Developers can rely on Leo to optimize and verify the generated circuits, ensuring that they are correct, efficient, and secure. Users can benefit from faster and cheaper zero-knowledge proofs, without sacrificing privacy or security.
  • Security: Leo enhances the security of zero-knowledge applications by implementing a “private by default” safety model, without requiring any explicit encryption or hiding. Developers can use private types to declare private variables and functions, which are encrypted and hidden from the public by default. Users can trust that their sensitive data and logic are always protected by zero-knowledge proofs, without requiring any third-party verification or audit.

Conclusion

Leo is a new programming language for private applications on Aleo. It is a functional, statically typed language that abstracts low-level cryptography and makes it easy to express logic in zero-knowledge. Leo offers developers a high-level, blockchain-native syntax, robust error handling, and a “private by default” safety model, combining performance, reliability, and security in blockchain development.

--

--

Prateek Tripathi

Introducing Prateek Tripathi, a tech-savvy individual with a passion for all things crypto, blockchain, and coding.